Manufacture of alloys



Patented Feb. 14, 1928.

UNITED STATES VERE B. BROWNE, OF BBACKENBIDGE, PENNSYLVANIA.

MANUFACTURE OI ALLOYS.

No Drawing.

This invention relates to the manufacture of alloys, and especially tothe manufacture in the electric arc furnace of ferrous-alloys, commonlycalled alloy steels.

In the production of suchal1oys, and especially those of the type ofrustless 1ro n, stainless steel and the like which contain chromium inproportions above about 10 or 12 per cent, it is of great importancethat the metal should be. free from oxids and likewise substantiallyfree from dissolved gases. The necessary freedom from oxids is usuallyattained by removing the oxidizing slag under which the metal has beenmelted down and refined, and replacing it bya basic slag which, as afinal operation, is reduced or brought to a reducing condition either bycarbon or by silicon or ferrosilicon. \Vhen lime slags are employed, asis usual ractice, these reducing agents form therewith calcium carbideand calcium silicate, respectively.

Calcium carbide slags are generally employed when the carbon contentofthe metal is not of primary importance, or when the carbon specificationof the metal is such as to permit a fairly large absorption of carbon bythe metal from the slag. When, on the other hand, carbon specificationsmust be maintained at a low limit, ferrosilicon is commonly employed asthe reducing agent. In this way metal 'of the desired low carbon contentmay readily be made, but other and often serious difficulties are apt toarise.

For example in the productlon of rustless iron, where a chromium contentof about 12%'-with a carbon content of 0.12% or less is required, theuseof carbide slags is usually.

excluded by the carbon specification. Ferrosilicon has therefore beenextensively used; but the product has been seriously affected by certainfactors which are directly \a-ttributable to its action. In the firstplace, as the oxids of iron, manganese, chromium, etc. are reduced backinto the metal there is a corresponding formation of silica, SiO,, whichcombines directly with most oxids yielding silicates, many of which areirreducible at the temperature of the steel-making furnace andconsequently remain in the slag.

In the second place a portion of the silicon, on account of its highspecific gravity, enters into and alloys with the metal during thereduction of the slag. The metal at this stage of the operation ishighly saturated with gases; and silicon, even in proportionsApplication filed October 21, 1925. Serial No. 64,015.

as low as 0.1% or less, greatly increases the solubility of these gasesin the steel at high temperatures. Consequently the gases cannot beeffectively removed from the molten metal, but are given up durinsolidification in the molds. In exaggerate cases this may give rise torismg or bleeding ingots; or apparently sound ingots may be formed whichnevertheless contain innumerable small blow holes, which cause the ingotto crack during rolling, resulting in heavy losses and high operatingcosts.

According to the present invention these difliculties are whollyovercome, and I am enabled to produce sound ingots which can be rolledwith only such commercial losses as would be encountered in the case ofordinary sound steels.

In practicing my process it is essential first, that the metal to besubjected to the reducing slag should be very low in or practically'freefrom silicon; and second, that this secondary or reducing slag should beof such a character, and used in conjunction with such-reducing agents,that no substantial quantity of silicon can enter the bath until thedeoxidation step, as hereinafter described, has been completed. I usealuminum as my preferred reducing agent, but I use it in conjunctionwith a substantially silicate-free slag, whereby injurious contaminationof the metal bath with silicon is avoided.

My preferred procedure is as follows:

I first subject the metal bath to any of the usual oxidizing slags,continuing this treatment until preliminary tests show that the metal issubstantially free from silicon, which should not in any case exceed afew hundredths of one per cent, and is preferably wholly absent. I thenremove the oxidizing slag, and replace it by a protective coating ofburnt lime. which should contain little or no silica. The use of fiuorspar additions, such as are commonly employed to increase the fusibilityof lime slags, should be avoided, especially in case the walls or roofof the furnace contain silicates, for the reason that the attack uponsuch walls b the fluorides is of a character which wil lead eventuallyto the contamination of the bath with silicon. A small addition ofmanganese ore however may be and preferably is made, for a purpose whichwill appear hereinafter. By reason of the high melting alumina or amixture of these, the mixture containing the aluminum powder being asintimate as possible in order to protect the aluminum from prematureoxidation. The aluminum reduces the oxides present, including thosederived from the metal bath; and the small amoiint of manganese oxid. ifused, serves to initiate and accelerate this reaction. As the aluminumis added the protective coating becomes progressivel more and morefusible, until finally there 1s formed a highly fluid slag whichexhibits a remarkable afiinity for the oxid contents andh also thesulfur contents of the metal While the nature of the chemical andphysical action cannot as yet be clearly defined, it is noticeable thatfollowing each addition of the aluminum-lime mixture, up to a certainpoint the slag darkens rapidly, owing to absorption of oxids from thebath, such darkening becoming less and'less and finall disappearing whendeoxidation is comp etc. It is apparent that this slag has a muchgreater affinity for the oxids than has the metal bath, with the resultthat the oxids, as well as the sulfur compounds, migrate rapidly intothe slag. Further additions of aluminum are made until the removal ofoxids from the metal is complete, a condition which manifests itself ina very characteristic manner, which permits the process to be controlledwith a degree of exactitude impossible with any'other slag with which'lam familiar.

The first manifestation occurs when a metal rod which has been dippedinto the slag is plunged into water. A strong evolution 0]. gas occurs,amounting at times almost to an explosion, and causes the slag to fluffup into a feather mass resembling snowflakes. This mani estation appearsonly toward the completion of the deoxidation; and I have found byexperience that when this condition has persisted for a period of aboutten minutes, it is quite safe to tap the metal, which will be found freefrom oxids, low'in sulfur, and of a composition to roll satisfactorily.For convenience and brevity, I will hereinafter refer to this phenomenonsimply as the test condition.

I have observed that when the slag has reached the test condition abovementioned, the mass of slag in the furnace appears to be frothing,giving out large volumes of gas which burn with a bluish flame. This gasmust of necessity be derived from the metal.

When the slag is dropped into water the evolved gases smell strongly ofhydrogen sulfid, indicating the desulfurizing nature of the operation.This is confirmed by the analytical results. In one specific case thesulfur was reduced from .044 to 028% in the space of twenty minutes: andhigh grade alloys prepared by this process will run consistently with asulfur content of .Ol to 018%. a result not attainable from the samematerials by the silicon reduction method as ordinarily practiced.

As already pointed out, an essential feature of the process is to keepthe content of silica in the slag as low as practicable, because thealuminum will reduce the silica to silicon, which passes to the metalbath and produces the same phenomena which occur when silicon is used asa reducing agent in the ordinary manner: in other words, aluminum, inpresence of a siliceous slag, acts essentially like silicon. This factis believed to account in large measure for the many failures which haveattended the use of aluminum in the past, because it has been the customin all such cases to add either sand or fiuor s ar to the secondary limesla before the reduction, to improve its fusibility. In case sand isadded, silicon will be reduced directly into the metal. In case fiuorspar is used, as is the normal practice in most steel melting shops, thefluorin which is evolved attacks the brick lining of the furnace withformation of silicon tetrafluorid, which is taken up by the slag, andbreaks up in turn with liberation of more fiuorin, thus repeating thecycle indefinitely; so that even relativel small additions of fluor sparmay eventual y result in slags which are quite high in silica. It shouldbe understood however that all precautions to keep the second ary slaglow in silica ma be futile, unless the metal to be deoxidize is very lowin or substantially free from silicon; and my rocess contemplates theapplication of a uminum reduction, in a substantially silicatefree slag,overlying a metal bath low in silicon.

My process is applicable generally to the preparation of ferrous alloyssuch as carbon steels or alloy steels, including such as contain siliconin any proportion: but in this case the process of deoxidation iscarried out in absence of silicon, as already'described, the siliconbeing introduced in any desired proportion after the completion of thisste In the process as described above it will be observed that thealuminum powder performs two quite distinct functions: it reduces theoxid components of the lime slag and thereby provides a reducing slagwhich deoxidizes the metal bath; and it forms with the lime a fusiblealuminate slag which ra idly absorbs both oxids and sulfur compoun sfrom the metal. Obviously the first of these functions might beperformed by other silicon-free reducing agents, including carbonrequired to construct a suitable aluminate slag should be supplied fromthe relatively expensive aluminum powder; but I may make a syntheticlime-alumina slag and add thereto only such amount of metallic aluminumas is required for deoxidizing purposes. This is in efi'ect what isaccomplished when the aluminum powder is added in admixture withalumina, instead of with lime, as already mentioned.

I I claim:

1. In a process of making ferrous alloys, subjecting a metal bath low insilicon but containing oxids to the action of a basic slag substantiallyfree from silica, and imparting a reducing character to said slag anddeoxidizing the metal bath by means of aluminum.

in silicon but containing oxids to the action of a basic slagsubstantially free from silica, and imparting a reducing character tosaid slag and deoxidizing the metal bath by means of aluminum.

3. In a process of making ferrous alloys, the steps comprisingdeoxidizing a. metal bath low in silicon in presence of aluminum and. abasic slag substantially free from silica, and tapping the metal.

4. In a process of making ferrous alloys, the steps comprisingdeoxidizing a metal bath low in silicon in presence of an aluminate slagsubstantially free from silica, and tapping the metal. 7

5. Process of makin iron-chromium alloys, com rising deoziidizing alow-silicon al- 10y melt y means of alummum in presence" of a basic slagsubstantially free from silicaifz- Y 6. Process of makin iron-chromium8.1- loys, comprising deoxidlzing a low-siliconalloy melt y means ofaluminum in presence o ilf an aluminate slag substantially free from s1me.

In testimony whereof, I aflix my signature.

-. VERE B. BROWNE-

